1. Field of the Invention
This invention pertains to certain compositions of matter and the use of such compositions in the hydraulic fracturing of subterranean formations penetrated by a borehole (wellbore). More specifically, this invention relates to compositions of matter and a method of controlling the direction of growth of fractures created during hydraulic fracturing.
2. State of the Art
Hydraulic fracturing is a well known stimulation technique used to recover formation fluids (e.g. oil and gas) from subterranean formations. In hydraulic fracturing, a fracturing fluid is injected through a wellbore to the formation to be treated at a rate and pressure at least sufficient to initiate and/or extend a fracture into the formation. Generally, a "pad fluid" is injected initially to establish formation breakdown (i.e., to initiate the fracture) and injectivity. The fracturing fluid usually carries a proppant into the fracture to hold the fracture open once the external pumping pressure is relieved. The physical requirements placed on the pad fluid and the subsequent fracturing fluid are, in many instances, different and the fluids are optimized according to their intended function.
The fracture(s) created during hydraulic fracturing can take different configurations and orientations, relative to the wellbore. Even though considerable information is available to the service company engineer designing the fracturing treatment from well logging, field experience, and core analysis, most attempts to control the fracture have been limited to a selection of the zone to be fractured, the point of injection into the zone, and selection of the treatment fluid and pump rates. This results in a fracture geometry determined primarily by the prevailing geologic stresses in the formation treated, supplemented by the inherent strength of the formation and its composite make-up. If the operator is fortunate the fracture proceeds horizontally within the pay zone. In many instances, however, the fracture proceeds vertically.
It is desirable to limit and control the extent of vertical fractures. Unless vertical fractures are limited and controlled, there is a possibility of fracturing out of the pay zone and into a barren zone or into a zone bearing undesirable fluids (e.g. water or brine) which could cause formation damage and contamination of fluids in each zone. In addition, fracturing fluids consumed in initiating and/or extending such unwanted vertical fractures are wasted.
There have been some prior attempts to control fracture geometry and/or to limit vertical fractures. For example, Hanson et al. (U.S. Pat. No. 3,151,678) described a fracturing process in which various rates of injection were used and the proppant in the fluid acted both as a proppant to prevent closure and as a semi-permeable plug which closed off paths of least resistance. Hanson (U.S. Pat. No. 3,159,217) also attempted to achieve more effective fractures by including in the fracturing fluid certain solid plastically deformable charges or slugs having a density equal to or different from the treatment fluid. Hanson said the density of such charges could be increased by adding heavier weighting materials (e.g. BaSO.sub.4 and Fe.sub.2 O.sub.3) or decreased by adding lighter weighting materials (e.g. cellulose or small hollow bodies, i.e., glass beads). In an attempt to control the direction of the vertical fracture up or down, Prater (U.S. Pat. No. 3,372,752) simultaneously injected two or more fracturing fluids of different densities along with a proppant that matched the density of one of the fluids, e.g., water, and a light oil containing aerated glass beads having a specific gravity of about 0.7 (column 3, lines 3-19). Ceramics having internal voids was described by Harold L. Graham and Othar M. Kiel (U.S. Pat. No. 3,399,727) as a proppant suitable for partial monolayer propping in fracturing treatment. Included under the term ceramics were particles of glass, porcelain, or alumina containing gas bubbles.
Braunlich (U.S. Pat. No. 3,335,797) taught a method for controlling the downward growth of a vertical fracture by a multiple step fracturing process which selectively placed a heavy proppant in the lower portion of a fracture and diverted subsequent fluids upwardly and horizontally. The concept of "similarly" preventing upward growth of vertical fractures using a bouyant material instead of the heavy proppant has been a topic of primary academic concern. See, for example, SPE 9260 (1980); SPE/DOE 11637 (1983); and seminar paper SPE/DOE 11606 (1983) presented by J. D. McLennan et al.